Paper product and method of making the same



Patented May 7, 1946 PAPER PRODUCT AND LIETHOD OF MAKING THE SAMEKenneth W. Britt, Norwood, Pa., asslgnor to Scott Paper Company,Chester, Pa., a corporation of Pennsylvania No Drawing. ApplicationAugust 13, 1941, Serial No. 406,727

11 Claims.

- water to produce an interfelted web. A considerable amount of adhesionor bonding between the individual fibres in contact with each otherdevelops upon drying this web, and it is this peculiar property ofcellulosic fibre that accounts for the strength of paper. In someinstances, the dry strength, tear resistance, and other characteristicsof the dry sheet are not as high as is desired, and the presentinvention afl'ords a means by which these properties may be markedlyincreased.

In addition, the lack of wet strength of the usual paper sheet isdisadvantageous in various types of paper, and is particularly marked inthe case of products which during their use come into contact withmoisture, for example, it is espec-ially disadvantageous in a papertowel, the normal use of which takes place in the wet or partially wetstate. results from the loss of most of the normal interfibre adhesionwhen the paper becomes wetted with water; for example, the usualabsorbent paper retains only 5% to 10% of its dry strength when wet. Thepresent invention provides a paper in which the interfibre adhesion orcementation in the sheet has been modified so that a paper is obtainedwith a high degree of wet strength and of resistance to rubbing in thewet state, as compared to an untreated paper sheet. The paper alsopossesses a high degree of resistance to strength loss when exposed tomoist conditions for long periods of time and tends to recover any losswhich it may have suffered when again restored to dry conditions.

Other desirable properties may be imparted to paper by the presentinvention, including increased absorbency, and decreased shrinkage andexpansion when exposed to water or water vapor. The present inventionalso aiiords a means whereby a weak, porous waterleaf paper may beconverted into a sheet simulating bond or parchment in appearance andfeel. A paper may also be produced in accordance herewith which is bothstronger and softer than a paper of the The lack of wet strength.

same type not treated in accordance with the invention.

The principal object of the invention is to provide a paper whoseproperties are advantageously modified as hereinabove pointed out, thecharacteristics imparted to the paper sheet in any particular instancebeing dependent on the use for which the paper is intended.

Another object of the invention is to provide a paper of improvedcharacteristics, which paper may be easily and economically preparedwithout the use of any toxic or other chemical detrimental in theintended use of the paper,

A further object is to provide a paper, such as towel paper, having arelatively high degree of wet strength without impairment of suchproperties as water-absorbency and softness required therein.

Other objects, including the provision of a novel method of preparingthe product of the invention, will be apparent from a consideration ofthe specification and claims.

In accordance with the present invention, a paper having wet strength inexcess of that inherent in the normal paper structure, and otherimproved properties, is provided in which the increase in wet strengthis imparted to the paper by the presence of silica within and upon thesurface of the fibres, said silica being distributed throughout theinterfelted structure of the paper. The product is prepared byassociating silicic acid (SiOanHzO) with the interfelted structure of apaper sheet so that it is within and upon the surface of the fibresthroughout the interfelted structure of the paper, and the treated paperis heated to convert the silicic acid into silica. The heating stepserves to dehydrate to some extent at least the hydrophilic silicic acidand to convert it into the insoluble silica. The term silica is usedherein to refer to the condition of the silicic acid after the paper hasbeen dried, and is not used in a narrow sense, since the silica in thefinished paper sheet may be in a somewhat hydrated form. The silicicacid is associated with the paper in such an amount that the silica,determined as $102, present in the finished paper, is suflicient toimpart desirable properties such as wet strength to the paper, but inamounts insufiicient to render the paper unsatisfactory for its intendedpurpose. It has been found that as low as 0.1% S102 by weight in thefinished paper based on the dry weight thereof will increase the desiredproperties of the finished sheet, including the wetstrength thereof, toa measurable extent as compared to the characteristics of the same papernot containing silica as herein described. In general, as the amount of8102 present. is increased, within limits, the properties of the paperare enhanced. However, due to the fact that desirable characteristics ofthe paper, such as the dry-tear resistance and elasticity, tend to bedecreased when the S: present in the finished paper exceeds about 8%,the limits of the S102 present in the Paper, in accordance with thepresent invention, are between 0.1% and about 8% by weight, based on thedry weight of the paper.

The amount of silica employed in any particular instance will dependupon the paper treated and the properties desired in the finishedprodnot.

In the treatment of towel paper to increase its wet-strength andresistance to rubbing when wet, it is essential to maintain the normalcharacteristics of the paper, such as softness, for otherwise theproduct would not serve satisfactorily for its intended purpose.Therefore, the upper limit of S10: will be controlled so that the paperis not appreciably altered in such characteristics, and the amount ofS102 present will generally be between 0.1% and about 3.0% by weightbased on the dry weight of the paper, preferably between about 0.5% and.0%, generally about 1%. As will hereinafter appear, if in anyparticular case undesirable stiffness is imparted to the sheet by reasonof the presence of the silica therein, a dispersible oil may beincorporated in the product to impart softness thereto.

The present invention is to be differentiated from the silicate sizingprocess, wherein sodium silicate solution is added to the dilutepulp-water suspension in the beater, followed by the addition of alum.The alum and sodium silicate react to form a precipitate, a portion ofwhich adheres to the surfaces of the fibres, without significantpenetration thereof. In accordance with the present invention, the paperafter its formation into an interfelted web is treated in such a mannerthat silicic acid is incorporated within, as well as upon the surface ofthe fibres in controlled amounts to impart to the paper the desirablecharacteristics. The incorporation of the silicic acid within and uponthe surface of the fibres, as herein contemplated, produces a sheetthat, upon drying, has silica deposited within the fibres which reducestheir tendency to swell upon the application of water and otherwisefunctions to impart the desirable properties described.

The paper treated in accordance with the invention may be of any type orgrade, for example, towel, waterleaf, kraft, bond, wall board, buildingpaper, outdoor advertising paper, photographic paper, wrapping paper,and the like.

As previously stated, silicic acid is associated with the interfeltedstructure of the paper so that it is within and upon the surface of thefibres thereof, and the treated sheet is heated to convert the silicicacid into silica. The silicic acid may be associated with the paper byseveral methods, for example, by treating the paper with an aqueoussolution of silicic acid, or by treating the paper with a solution of analkali metal silicate, with subsequent neutralization, either before orafter drying, of the alkali metal oxide associated with the $102 of thealkali metal silicate, so that there is no sodium silicate present, thatis, no sodium oxide associated with the SiO2. As a result of theneutralization, a siliceous product is formed, which for the purposes ofthe present invention is deemed to be silicic acid.

The impregnating solution is brought into contact with the paper eitherduring its fabrication or after it has been fabricated and dried.Advantageously, the solution is applied to the paper while the papersheet is being processed, and the solution may be brought into contactwith the paper sheet at any stage, after the formation of theinterfelted fibre web and prior to the completion of the drying thereof,where sufllcient water has been removed to permit absorption of thesolution by the sheet. Preferably, the treatment with the impregnatingsolution takes place when the moisture in the sheet has been reduced toabout 30%, or below, and, therefore, at a suitable point in the driersection. The passage of the paper over the remaining driers will besuflicient to dry the paper and to deposit the chemical within and uponthe surface of the fibres. In the case the solution employed is asolution of silicic acid or in the case paper has been treated withsodium silicate solution followed by treatment with the acidic materialprior to drying the paper, silicic acid is deposited in the paper whichis converted to silica upon drying. As will hereinafter be discussedfurther, in the event the acid treatment is not carried out until afterthe paper is dried, alkali metal silicate will be deposited within andupon the surface of the fibres prior to the treatment with the acidicmaterial. In the case a dried paper sheet is treated with theimpregnating solution, the sheet after treatment with the solution ofsilicic acid or with the solution of alkali metal silicate followed bythe acid treatment may be dried in any desired manner by subjecting itto a temperature above 212 F. and below the temperature at which thepaper is damaged, for example, at 250 F. to 300 F., in a drying chamberor by passing it over a heated roll or through a heated,

calender.

The paper sheet, whether treated during the paper-making process orafter drying, may be brought in contact with the solution (or solutions)by any suitable means in order to impregnate the paper. For example, itmay be applied by means of a size press such as is used in theconventional tub sizing process. Alternatively, it may be sprayed uponthe moving sheet of paper by means of a spraying device.

An aqueous solution of an alkali metal silicate is used in thepreparation of the silicic acid solution if the paper is to be treatedtherewith and is used to impregnate the paper in the process wheresubsequent treatment with acid forms the silica in the paper sheet. Thesolution has been designated herein as an alkali metal silicate solutionwhich includes both potassium and sodium silicate, but since the sodiumsilicate solutions are more economical for use, the invention will bedescribed hereinafter using sodium silicate as typical. While a solutionof any desired ratio of NazO to SiOz, where the molecular ratio of S102exceeds that of NazO, for example, solution having a molecular ratio ofNazO to SiOz between 1 to 3.8 and 1 to 2, may be employed, the use of asolution relatively low in alkalinity is preferred, since less acid isrequired for its neutralization. In addition, in the process in whichthe paper is impregnated with the sodium silicate solution, prior to theacid treatment, it is possible by the use of a solution relatively lowin alkalinity to associate a larger amount of SiO2 with the paperwithout encountering difficulties due to excessive the paper isimpregnated with sodium silicate prior to the acid treatment, a sodiumsilicate solution having relatively high alkalinity may be adjusted tothe desired alkalinity by the addition of appropriate acidic material.

to neutralize the alkali, and to produce a solution of the desiredacidity and concentration, The

choice of acidic material for reaction with the silicate is notimportant, and among the materials available for use, the following maybe mentioned: hydrochloric, sulphuric, nitric, phosphoric, acetic,formic, oxalic, and lactic acids, and sodium bisulphate. Mixtures ofacids may be employed, if desired. Ver satisfactory results are obtainedif sulphuric or hydrochloric acid, or a. mixture of hydrochloric andacetic acids, is employed. 1

In the preparation and use of the silicic acid solutions, care must betaken to prevent gelation. The time of gelation is dependent principallyupon the pH value of the solution, the concentration of silicic acid andthe temperature. On the acid side of the neutral point, the higher thepH, the more rapid the gelation; also the higher the concentration orthe temperature, the more rapid is the gelation. The pH value of thesolution applied to the paper should not be so low that a detrimentaleffect upon the cellulosic fibre results, and the concentration ofsolution and the amount of solution applied to the paper must be soadjusted as to deposit the desired amount of silica in the paper. Anadditional reason for employing a solution of only mild acidity residesin the fact that, in most instances it will be desirable to provide afinished paper which is only slightly acid, for example, a paper with apH value between about 3 and 6.5, preferably between 4.0 and 6.0, asdetermined by applying to the dry paper drops of standard La Motteindicator solutions and noting the color.

In View of these facts, the range of pH of the solution as applied tothe paper should be from about 2 to 5, preferably between 2.5 and 3.5,and the concentration of silicic acid should not exceed about 10%,figured as S102 and generally above 1%, preferably between 4.0% and7.0%. The solution of silicic acid should be maintained at temperaturesbelow about 30 C. It is understood that the higher concentrations ofsilicic acid are used in conjunction with the lower pH values, while thehigher pH values may be safely used only with the lower concentrationsof silicic acid.

There are three method that may be employed in handling the silicic acidsolutions: (1) the preparation in advance of a quantity of silicic acidsolution suitable for application to the paper; (2) the meteringtogether continuously of acid and silicate solutions of suitableconcentration immediately before application to the paper; and (3) thepreparation of a relatively stable silicic acid solution of pH value of1.0 to 2.0 and the metering of this solution with suflicient silicatesolution immediately befor application to the paper to raise the pH tothe desired point for such application. It is important to have rapidand complete mixing during the preparation of the silicic acid solution.

In the first case, the upper safe limit of silicic acid concentration isabout 5% and the preferable range is from 2% to 4%, and the pH range ispreferably from 3 to 4. In the preparation of the silicic acid solution,it is desirable to add the diluted sodium silicate solution to thedilute acid, rather than the reverse. Again, it is understood that thehigher concentrations are preferably used with the lower pH values,while the higher pH values may be safely usedwith the lowerconcentrations. In'the use of this procedure, the amount of solutionapplied to the paper will generally be large, since otherwise only verysmall amounts of silica would be deposited in the paper.

In the second case, the pH value of the mixture of acid and silicate ispreferably about 2.5 to 3.5, and the concentration of silicic acid maybe as high as 10%, or even higher, preferably about 4% to 7%. It is seenthat in this case the amount of solution applied to the paper may berelatively small, since the higher concentration permits the depositionof relatively large amounts of silica that will occur even withrelatively small amounts of solution.

In the third case, the very acidic silicic acid solution, pH 1.0 to pH2.0, may be prepared in concentrations as high as about 8%, and asilicate solution or suitable buffer solution is metered into thissolution to form a solution of the desired pH value and concentration.The final mixture as applied to the paper will conform to the limitsgiven under the second method.

In all cases, it is advantageous to keep the temperature of allsolutions as low as feasible. For example, after the preparation ofdilute sulphuric acid, it is advantageous to allow the heat produced bythe dilution to dissipate before the addition of the silicate. In someinstances, it may be desirable, in order to prevent gelation, torefrigerate the solution to a temperature below room temperature, and,even to a temperature only slightly above the freezing point of thesolution.

In the modification of the process where the paper is treated with asolution of an alkali metal silicate with subsequent neutralization.preferably by treatment with acid, to precipitate 'the silica, theconcentration of the silicate solution is relatively unimportant so longas the solution is sufficiently dilute to be absorbed by the paper, anexample of a range of concentrations (solid sodium silicate in solution)which may be employed being from .5% to 10%, preferably from 2 to 4%,although it is to be understood that solutions of other concentrationsma be used.

Preferably, the treatment with acid follows the treatment with thesilicate solution without any intermediate drying step, for in such casethe precipitation of silicic acid within and upon the surface of thefibres of the interfelted web takes place rapidly and completely. Ifdesired, however, the paper after its treatment with the silicatesolution may be dried and subsequently the acid solution applied. Forexample, a paper containing sodium silicate, with or without silica,within and upon the surface of the fibres may be treated with an acidicmaterial to neutralize the sodium silicate, the amount of siliceousmaterial in the paper providing the silicic acid, and,

upon drying, the silica required to give the desired characteristics tothe paper. The amount and concentration of the acid solution applieddepends on the alkalinity of the paper following the treatment with thesilicate solution, and sufiicient acid is added to convert the alkalimetal silicate into silicic acid, but in order not to damage thecellulose fibre and to provide a paper of relatively low acidity, theuse of a large excess of acid is avoided. Any acidic material may beemployed and those hereinbeiore mentioned for use in the preparation ofthe silicic acid solution are typical examples, but organic acids, suchas acetic or formic, have been found to be most advantageous. Forexample, the concentration of the acid, in the case of acetic acid, willbe between .2% and 6.0%. As previously stated, the acid may be appliedin any desired manner, for instance, by a tub sizing step or byspraying.

An alternative, but less preferable, method of neutralizing the NaOzassociated with the S10:

of the sodium silicate resides in the incorporation in the paper of anammonium salt, such as ammonium chloride, ammonium sulphate, ordiammonium phosphate, the latter being preferred, and the subsequentheating of the paper to volatilize ammonia, with the subsequentconversion of the silicate into silicic acid, and finally into silica.The ammonium salt is preferably incorporated in the paper simultaneouslywith the incorporation of the sodium silicate, but the paper may beimpregnated with the solution of the ammonium salt prior or subsequentto its impregnation with the sodium silicate solution.

"tity to contribute materially to the wet-strength of the paper. Theinvention of this application, as previously pointed out, contemplatesthe deposition of silica within and upon the surface of the fibres, andit is possible to accomplish this result by the use of a silicic acidsolution or by the subjection of paper containing alkali metal silicateto neutralization to form silicic acid. Subsequent heating in both casesconverts the hydrophilic silicic acid into silica.

The properties of the product of the present invention, whether made byemploying a silicic acid solution or a sodium silicate solution withsubsequent treatment with acid, may be further modified by theincorporation therein of other materials such as starch, glue, andvegetable gums. If desired, additional softness may be imparted to thepaper by the incorporation therein of a dispersible oil, for example, ofthe type known as a textile finishing oil. The termfdispersible oil asused herein and in the claims includes those oils which, either throughtheir emulsifiable or soluble properties, are capable of being dispersedin an aqueous medium. A paper containing silica within and upon thesurface of the fibres, if treated with these oils, within the practicallimits hereinafter set forth, is considerably softened and yet retains astrength, both in the wet and dry state, substantially greater than thatof a similar paper so treated which does not contain silica. Since theuse of the dispersible oil tends to cause a decrease in the strength ofthe paper, the amount thereof associated with the paper will be keptbelow the point where the increase in strength imparted by the silicawill be unduly reduced by the oil. Satisfactory results are obtained byusing about .1% to .5% of oil by weight based on the dry weight of thepaper, and the use of amounts higher than this and up to about 3%, oreven higher, may be used if desired, depending on the type of paper, theamount of silica associated therewith, and the results to be obtained.The dispersible oil may advantageously be applied to the paper byspraying on the paper or by dispersing it in the solution (or solutions)with which the paper is treated, or in any other suitable manner,

The following examples are illustrative of the process:

Example I parts of a 10% sodium silicate solution in which theproportion of SlOz to NazO is 3.8 to 1 is added gradually to 20 parts ofa 10% hydrochloric acid solution in parts of water. The

resulting solution contains about 3.5% silicic acid and has a pH valueof about 3. Towel paper weighing 34 pounds per ream prepared from 65%unbleached sulphite pulp and 35% mechanical wood pulp is sprayed orotherwise impregnated with this solution to a moisture content ofapproximately 20%. The paper is then thoroughly dried at a temperatureof about 2500 F. and the finished paper contains 375% S102. The wetstrength of the paper is increased to 3 to 4 times over that of theoriginal paper, and the other characteristics of the paper, such asabsorbency and dry tensile strength, are improved. The paper retains itssoftness and other properties required in a towel paper,

Example II 100 parts of a 10% sodium silicate solution in which theproportion of SiO: to NazO is 3.8 to l is added gradually to 20 parts ofa 10% hydrochloric acid solution in 30 parts of water. The resultingsolution contains about 6% silicic acid and has a pH value of about 3.0.This solution is applied to towel paper made from 50% bleached sulphatepulp and 50% unbleached sulphite pulp at a point in the paper machinedrier section at which the paper contains approximately 30% watercontent. The amount of silicic acid solution applied is 15% of the dryweight of the fibre and the weight of S102 retained by the paper isapproximately 0.7% of the fibre. The'paper is then thoroughly dried bycontinuing its passage through the drier section at a temperature ofabout 300 F. The wet strength of the paper is increased 3 or 4 timesover the original paper, and the other characteristics of the paper,such as absorbency and dry tensile strength, are improved. The paperretains its softness and other properties required in a towel p pExample III A waterleaf uncreped paper made from 100% bleached pulp ofhigh alpha content is impregnated with a silicic acid solution of 4%concentration and a pH of 3.5, the amount of solution taken up by thepaper being about 50% of the dry weight of the fibre. The amount of S102retained by the paper is 2%. The wet tensile strength is approximately 9times that of the untreated paper, and is greatly increased instillness, rubbing resistance, and surface hardness and resembles a goodgrade of bond paper in feel and appearance.

Example IV Two parts of a dispersible oil (Socony Vacuum CompanysTextile finishing oil B) are added to the silicic acid solution ofExample I and the paper is treated as set forth in that example.-

The finished paper contained 0.8% dispersible oil and was appreciablysofter than the paper obtained by Example I. The wet strength of thepaper, however, was not materially diminished.

Example V A solution of sodium silicate containing a ratio of NazO toSiOg of l to 3.8 is applied by spraying or otherwise to towel paperweighing. 34 pounds per ream prepared from 65% unbleached sulphite pulpand 35% mechanical wood pulp at a point in the paper machine driersection at which the paper contains approximately 30% water. The amountof sodium silicate solution applied is.20% of the dry weight of thefibre. The paper as soon thereafter as convenient and prior toappreciable further drying thereof is treated by spraying or otherwisewith a 3% solution of acetic acid, the acid solution application beingof the dry weight of the fibre. The paper is then thoroughly dried bycontinuing its passage through the drier section at a temperature ofabout 300 F. The weight of silica figured as S102 retained by the paperis approximately 0.8% of the fibre. The paper had properties similar tothose given above in Example II.

Example VI ,fipplied being 20% of the dry weight of the fibre.

The paper as'in Example V contains approximately 0.8% of silica figuredas S10: and the paper had characteristics similar to those described inExample II.

The following table illustrate the properties Table I .Wet tensilestrength The wet tensile strength was determined on a Schopper tensilemachine and the results are in pounds tensile (machine direction)required to break a strip of wet paper 15 mm. wide.

Percentage of SiO: present Wet tensile strength Table II.-Poroaity Theporosity wa determined by a Gurley porosity tester and the figures areexpressed in seconds required to pass 300 cc. of air.

Percentage oi Number of B10, present seconds These figures indicate thatno appreciable change in porosity is encountered with the givenpercentages of silica.

Table IIL-Tearing strength Percentage of Tearin S10; present strengtThis table shows that when amounts of $102 less than about 8% arepresent in the paper, the tearing strength is increased. When largeramounts of SiO: are present, the tearing strength tends to be decreasedbelow the original paper; for example, at 10.9% S102, the tearingstrength is 73.4.

Table IV.-Sti1fnelss The stiifness was measured by a Clark softnesstester as described in Paper Trade Journal, March 28, 1935, and theresults are expressed as WLF/10 in which W is the basis of the paper ingrams per square meter and L is the critical length as determined by theinstrument.

Percentage of S10: present Shine Table V.Dry tensile strength- The drytensile strength was determined on a Schopper tensile machine, and theresults are in pounds tensile (machine direction) required to break astrip of dry paper 15 mm. wide.

I toga i tens 33m; s t angtli 0 2.10 1.24 2.16 2.46 2.24 no a. 12 7.503.7.5

Table VI In the following table, the paper was a towel paper weighing 34pounds per ream prepared from 65% unbleached sulphite pulp and 35%mechanical wood pulp. The effect of the use of a softening oil '(SoconyVacuum Company's "Textile finishing oil B") on the physicalcharacteristics of a paper is illustrated, the figures given beingdetermined as set forth in connection with the other tables:

Per eut Per cent Btiil'ness on Dry tensile Wet tensile WL While theabove tables set forth figures obtained by using towel paper,corresponding changes in properties are obtained to mor or less the sameextent in other types of paper. It

I will be seen that in accordance with the present invention, the amountof SiO: incorporated in the paper sheet will be determined in anyparticular case by the characteristics it is desired to impart to theproduct.

Considerable modification is, therefore, possible in the amount of SiOzpresent in the paper, as well as in the method employed in depositingthe silica within and between the fibres of the interfelted web, withoutdeparting from the essential features of the invention.

I claim:

1. A paper having increased wet strength and substantially unimpaireddry tear resistance, and having heat-treated silica within and upon thesurface of the fibres throughout the interfelted structure of said paperand present in an amount between 0.1% and about 8% SiO2 by weight basedon the dry weight of the paper, the said silica being the onlysilica-containing material present in sufiicient quantity to contributmaterially to the wet-strength of the paper.

2. A paper having increased wet strength and substantially unimpaireddry tear resistance, and having heat-treated silica within and upon thesurface of the fibres throughout the interfelted structure of said paperand present in an amount between 0.1% and about 3% SiO2 by weight basedon the dry weight of the pap r, the said silica being the onlysilica-containing material present in sufiicient quantity to contributematerially to the wet-strength of the paper.

3. A towel paper having increased wet strength and substantiallyunimpaired dry tear resistance,

and having heat-treated silica within and upon the surface of the fibresthroughout the interfelted structure of said towel paper and present inan amount between approximately 0.5% and about 2% SiO2 by weight basedon the dry weight of the paper, the said silica being the onlysilicacontaining material present in sufficient quantity to contributematerially to the wet-strength of the paper.

4. The method of increasing the wet strength of paper while preservingsubstantially the dry tear resistance thereof, which comprisesassociating within and upon the surface of the fibres throughout theinterfelted structure of said paper silicic acid in an amount sufficientto furnish between 0.1% and about 8% SiOz by weight based upon the dryweight of the paper, the said silica being the only silica-containingmaterial present in suflicient quantity to contribute materially to thewet strength of the paper, and heating said Paper to convert saidsilicic acid into silica, withinand upon the surface of the fibresthroughout the interfelted structure of said paper.

5. The method of increasing the wet strength of paper while preservingsubstantially the dry tear resistance thereof, which comprisesimpregnating paper with a solution of silicic acid in an amountsufilcient to furnish between 0.1% and about 8% S102 by weight based onthe dry weight of the paper at a point in the production of said paperafter the formation of the interfelted web and prior to the completionor the drying thereof, and heating said paper to dry it and to convertsaid silicic acidinto silica distributed within and upon the surface ofthe fibres throughout the interfelted structure of the paper.

6. The method or increasing the wet strength of paper while preservingsubstantially the dry tear resistance thereof which comprisesimpregnating paper with a. solution of sodium silicate, impregnatingsaid paper with a solution of an acidic material to convert the sodiumsilicate absorbed by the paper into silicic acid, said solution ofsodium silicate furnishing between 0.1% and about 8% SiOz by weightbased on the dry weight of the paper, and heating said paper to convertsaid silicic acid into silica distributed within and upon the surface ofthe fibres throughout the interfelted structure of the paper.

7. The method of increasing the wet strength of paper whil preservingsubstantially the dry tear resistance thereof which comprisesimpregnating paper with a solution of sodium silicate, impregnating saidpaper with a solution of an acidic material to convert the sodiumsilicate absorbed by the paper into silicic acid, said solution ofsodium silicate furnishing between 0.1% and about 8% SiOz by weightbased on the dry weight of the paper, said impregnating steps beingconducted at a point in the production of said paper after the formationof the interfelted web and prior to the completion of the dryingthereof, and heating said paper to dry it and to convert said silicicacid into silica distributed within and upon the surface of the fibresthroughout the interfelted structure of the paper.

8. The method of increasing the wet strength of paper while preservingsubstantially the dry tear resistance thereof, which comprises imprenating paper with a solution of silicic acid in an amount sufiicient tofurnish between 0.1% and about 8% SiOz by weight based on the dry Weightof the paper, and heating said paper to convert said silicic acid intosilica distributed within and upon the surface of the fibres throughoutthe interfelted structure of the paper.

9. The process of claim 8 in which the solution of silicic acid withwhich the paperv is impregnated has a pH value between about 2 to 5.

10. The process of claim 8 in which the solution of silicic acid withwhich the paper is impregnated has a pH value between about 2.5 and 3.5.

11. The method of obtaining a paper product of high wet-strength andsubstantially normal dry tear resistance which comprises neutralizing apaper containing an alkali metal silicate in an amount providing between0.1% and about 8% SiOz by weight based on the dry weight of the paperupon neutralization, said silicate being distributed within and upon thesurface of the fibres throughout the interfelted structure of saidpaper, and heating said paper to convert the silicic acid formed uponneutralization into silica.

KENNETH W. BRITT.

